When a component is subject to sufficient stress forces it reacts by undergoing dimensional change, referred to as “strain”. Stress and strain are related by the “Modulus of Elasticity” of the material of said component and accordingly, reference to one is indirectly a reference to the other.
Stress and strain have an indelible effect on the material at the microscopic and possibly even molecular level. For small amounts of stress/strain within the elastic range of the material, the effect of each stress/strain cycle is virtually negligible. Repeated stress/strain cycles have a cumulative effect causing the component to waken and eventually fail. The number of cycles to failure generally is a function of the amount of force (stress) applied. The greater is the stress per cycle the fewer are the cycles to failure.
Strain can be measured with a conventional electrical strain gauge. Such a gauge varies in resistance depending on loading (i.e. the amount of strain) but after loading it reverts to its unloaded state. Hence a strain gauge will only provide information on a given loading cycle rather than a history of the loading. Accordingly, a strain gauge on its own is not useful in predicting the remaining life of the component.
While a strain gauge could be augmented with a recording apparatus (for example a microprocessor) for recording loading history, this is generally impractical and in many applications impossible. For example if one wishes to monitor the loading history of a gear or a spring encased in a housing, there is no way in which to connect electrical leads to the strain gauge.
Accordingly it is an object of the present invention to provide an apparatus and a method for determining the extent of cyclic stress fatigue suffered by a component which doesn't require electrical connection or continued monitoring and tallying of each loading cycle.